This invention relates to an electrosurgical instrument and, in particular, to apparatus for converting a surgical instrument, such as a purely mechanical laparoscopic tool, for electrosurgical treatment
In the field of laparoscopic surgical tools, the ability to deliver radio frequency electrosurgical power for coagulation is well known. It is also well known for laparoscopic tools to provide for mechanical manipulation or a mechanical function. Tools combining these attributes include metal instruments with insulated shafts that can be used as monopolar instruments. There are several problems with such tools. Firstly, monopolar power delivery can cause excessive tissue damage as a result of the long conductive pathways. This is particularly so when attempting to deep perform deep coagulation which involves high curt delivery. A second problem is that such high current delivery stresses the electrical interface between tissue and electrodes, causing carbonisation and/or sticking. Yet another problem is that a metal instrument shaft can cause radio frequency energy to be capacitively coupled to entry ports or other closely coupled instruments.
One technique used in the prior art to overcome these problems is to use a bipolar instrument. In this case two electrodes are used to deliver electrical power to tissue. This localises the power distribution and lessens the danger of damage to adjacent structures. However, such instruments are more complicated particularly when movable elements are required such as for forceps or scissors. Further, the depth of effect is determined by the spacing of the electrodes from each other.
It is an object of the present invention to provide an improved instrument.
According to a first aspect of this invention, an electrosurgical instrument comprise: an elongate inner tubular member having a proximal end and a distal end, at least a portion of the member being electrically conductive in order that there is d.c. electrical continuity between the said ends; and an elongate tubular electrically conductive sheath surrounding the inner member but spaced from the conductive material of the inner member to form an electrically insulative jacket, whereby the member and the sheath constitute a coaxial transmission line, the sheath being electrically connected to the inner member at a location spaced proximally from the distal end; a feeder connected to the inner member distally of the connection between the sheath and the inner member, and an r.f. isolating structure between the said connections of the inner member to the feeder and to the sheath, the isolating structure presenting a series impedance between the said connections at an operating frequency of the instrument
The form of the isolating structure depends on the frequency of operation. In general, it is constituted by a resonant element or a resonant assembly of elements having a frequency of resonance corresponding to an operating frequency of the instrument. For comparatively high electrosurgical frequencies, typically upwards of 300 MHz, the isolating structure consists of a section of the inner member between the connections to the sheath and the feeder, the section having an electrical length of mxcex/4, where m is an odd number and xcex is the wavelength associated with the operating frequency in question. Since, at the proximal end of this section, the sheath is coupled to the inner member at least at high frequencies, the section operates as an impedance transformer presenting a high impedance at the connection with the feeder. At lower frequencies, humped components are preferred, such as an inductance formed by a section of the inner member between the connections to the sheath and feeder, this inner member section being surrounded by a body of high permeability material such as a ferrite ring. A resonating capacitor is coupled between a ground connection which may be the sheath, and a conductor of the feeder which is connected to the inner member, so as to produce a parallel resonant structure at the operating frequency.
It is possible to construct an isolating structure which isolates the proximal end of the inner member from the feeder connection at two widely spaced operating frequencies, for instance, at an upper operating frequency above 300 MHz and a lower operating frequency below 300 MHz. Typical frequencies are 2.45 GHz and 5 MHz respectively. In this case, two sections of the inner member which are electrically in series with each other constitute elements of respective resonant assemblies or unit at the upper and lower frequency respectively. The first section may extend between the connection of the inner member to the feeder and a decoupling capacitance between the inner member and the sheath, while the second section extends between the decoupling capacitance and a d.c. connection of the sheath to the inner member, this latter connection being the further from the distal end of the inner member. The first section has the quarterwave electrical length feature described above, while the second section is surrounded by high permeability material so that the two sections respectively provide the impedance transformer and series inductance for isolation at the upper and lower frequencies. It will be understood that the decoupling capacitance resents a low impedance connection between the sheath and the inner member at the upper frequency, but not at the lower frequency. At the lower frequency, the first section appears as a low series impedance.
The insulating jacket between the conductive material of the inner member and that of the sheath may perform a dual function, insofar as not only does it provide a dielectric medium of a coaxial transmission line comprising the inner member as an inner conductor and the sheath as an outer shield, but also it may serve as an optical path for illumination of the operation site adjacent the distal end of the inner member or for viewing of the site, the instrument acting as an endoscope. Thus, the space between the inner member and the sheath may be occupied by dielectric optical material such as glass or transparent plastics, formed as a tube, a rod, or as fibres. This optical material may also serve as a support for tile inner member within the sheath.
According to a second aspect of the invention, there is provided an electrosurgical instrument for electrosurgical treatment at an operating frequency of at least 300 MHz, comprising an elongate electrically conductive lumen housing a mechanically or optically functional element, an electrode at a distal end of the lumen and electrically coupled to the lumen, an elongate electrically conductive outer sheath coaxially arranged around the lumen and having a distal end adjacent the lumen distal end, the sheath being dimensioned to enclose an insulative layer (which may be air) such that the sheath and the lumen together form a coaxial transmission line, and an isolating structure associated with the distal end of the sheath to restrict the flow of electrosurgical current in the sheath
According to a second aspect of the invention, there is provided an electrosurgical tool converter for converting an elongate surgical tool into an electrosurgical instrument for performing electrosurgical treatment at an operating frequency of at least 300 MHz, the converter comprising an elongate electrically conductive lumen for receiving the tool with a working element of the tool exposed beyond an open distal end of the lumen to form an electrode electrically coupled to the lumen, an elongate electrically conductive outer sheath coaxially arranged around the lumen and having a distal end adjacent the lumen distal end, the sheath being dimensioned to enclose an native layer such that the sheath and the lumen together form coaxial transmission lint, and an isolating structure associated with the distal end of the sheath to restrict the flow of electrosurgical currents in the sheath.
The isolating structure may comprise a balun, typically a quarter-wave sleeve balun, arranged to yield a substantially balanced feed location in the region of the distal end of the sheath where, generally, a working clement of the instrument is attache, e.g. a pair of forceps jaws or scissor jaws.
In preferred embodiments of the invention, the is proximally coupled to the lumen in respect of electrical currents at the operating frequency, and the lumen itself is coupled to a fed structure at a location spaced axially from the coupling to the sheath. The axial spacing is such that, at the operating frequency, the lumen may be electrically short-circuited to the sheath, the sheath and the lumen being isolated from ten feed structure at that point at the operating frequency. Typically, the distance between the sheath-to-lumen coupling and the coupling of the feed structure to the lumen is a quarter-wavelength, the wavelength being the electrical wavelength of electrosurgical energy in the transmission line formed by the sheath and the lumen
The axial distance between the feed structure and each discontinuity in the sheath, particularly the distal end of the sheath, is preferably such that the corresponding electrical length is one half wavelength or a multiple thereof in order that the transmission line formed by the sheath and the lumen need not have the same characteristic impedance as that of the feed structure. A multiples xcex/2 structure yields a source impedance at the end remote from the feed structure corresponding substantially to the characteristic impedance of the feed structure. Indeed, it is possible to construct the transmission line so as to have more than one section of different characteristic impedances due, for instance to the sheath having portions of differing diameters. In such circumstances, each section preferably has an electrical length which is an integer multiple of xcex/2.
Delivering electrosurgical power at frequencies between 300 MHz and 100 GHz provides several advantages. The electrosurgical energy is propagated predominantly by dielectric means, which means that the conductive electrode/tissue interface is less important that at lower frequencies. By confining the electric field it is possible to define the treatment area in such a way that comparatively large areas may be treated with small electrodes. For cost benefit, particularly with regard to manipulable instruments such as forceps, the working elements can be constructed of a dielectric material with an embedded conductor rather than by more expensive metal fabrication techniques. UHF energy absorption, particularly at the ISM (Industrial/Scientific/Medical) frequency of 2450 MHz, is determined by water content, which means that variable tissue performance is virtually eliminated. For example, in conventional low frequency electrosurgery, performance is determined by conduction. Fat can have a conductivity which is a mere fraction of highly conductive body fluids such as blood or bile. The water content values, however, are not very different and UHF performance is consequently not as variable.
The proximal trap and the distal balun referred to above allow electrosurgical currents to be isolated to selected areas of the instrument at UHF operating frequencies. Thus, even though the instrument may have electrical continuity throughout its construction due, for instance, to the lumen and the sheath being conductive throughout their length, only selected areas carry electrosurgical currents and voltages. For instance, the instrument, or a combination of the instrument and an inserted non-electrosurgical tool may have an exposed metal handpiece or handle with electrical continuity to the treatment area without danger to the user. This also means that the tool may be made entirely of metal, including a rod actuator and the like within the lumen, and may, therefore, be made more robust.
The sheath is typically fed from a proximally located lateral aperture by a coaxial transmission line or, at frequencies above 5 GHz, a waveguide. The annular cross-section space between the lumen and the sheath may contain a dielectric medium, which may include fibre optics or rod lenses for illumination and visualisation, and fluid passageways for fluid delivery or extraction.
The selective coupling of UHF electrosurgical energy, as described above, allows conversion of standard laparoscopic instruments or endoscopes, for electrosurgical use on tissue The invention is also applicable to laparoscopic electrosurgical instruments. Thus, there may be provided, in accordance with the invention, an electrosurgical instrument comprising the combination of a tool converter as mentioned above and a surgical tool housed in the lumen, the tool having a working element projecting beyond the distal end of the lumen to form an electrode for electrosurgical treatment of tissue, and a handpiece projecting beyond a proximal end of the lumen.
The instrument may be adapted to operate additionally at comparatively low frequencies, i.e below 300 MHz and typically between 100 kHz and 40 MHz. In this case a choke may be placed around the lumen at a proximal end of the transmission line formed by the sheath and the lumen, to prevent conduction of low frequency electrosurgical currents proximally along the lumen or the shaft of a surgical tool housed in the lumen. The same feed structure may be used for delivering both low frequency and UHF electrosurgical energy. The choke forms a lumped impedance circuit, and the return path back to the r.f. source connected to the feed from the tissue is by way of stray capacitance between the patient and the return conductor, which may be referenced to ground potential.
The invention will be described below by way of example with reference to the drawings.